This invention relates to manufacturing olefin and/or diolefin polymers in fluidized beds, and particularly to detecting and correcting local defluidization and channeling in fluidized beds.
One of the most economical and commonly used methods to manufacture polyolefins is gas phase polymerization. Good fluidization conditions are essential for fluidized-bed polymerization reactors. The fluidized-bed reactors to which our invention relates may be quite large with typical production rates of about 12,000 lb/hr to as much as 150,000 lb/hr pounds per hour of polyolefin. Descriptions of xe2x80x9cUNIPOLxe2x80x9d fluidized-bed reactors (although our invention is not limited to them) may be found in U.S. Pat. Nos. 4,003,712, 4,877,587 and 4,933,149, which are incorporated herein by reference. A typical fluidized-bed reactor used in polymerizing olefins and/or diolefins will include a cylindrical section containing the dense-phase fluidized bed (i.e., the mixture of reaction gas and polymer particles), and an expanded section above the cylindrical section. Monomers and catalyst are introduced into the reactor, forming a small particulate product that becomes suspended in the fluidized bed which is maintained by the continuous introduction of gas through a distributor at the bottom of the bed. Gas is circulated through the fluidized-bed reactor to a heat exchanger to remove the heat generated in the exothermic reaction; the cooled gas is typically continuously returned in order to maintain steady conditions. Product is removed as needed through discharge port(s) usually located near the bottom of the fluidized bed.
In addition to well-known conventional gas phase polymerization processes, xe2x80x9ccondensing modexe2x80x9d, including the so-called xe2x80x9cinduced condensing mode,xe2x80x9d and xe2x80x9cliquid monomerxe2x80x9d operation of a gas phase polymerization can be employed; these involve the introduction of liquid to the fluidized bed or cycle gas stream. The introduced liquid evaporates quickly in the fluidized bed to help remove the heat of reaction. Uniform distribution of fluidizing gas and liquid in the cross-sectional area of the reactor is essential to achieve good fluidization in the bed. Good control over the fluidization conditions of this complex system is necessary to maintain desired production rate and product quality.
Under certain undesirable conditions, localized non-uniform gas and/or liquid distribution can take place in the reactor. A portion of the bed can have insufficient flow of fluid to fluidize particles, resulting in local defluidization. In normal fluidization, particles are suspended by the fluid and are in motion relative to other particles. In a defluidized zone, particles do not move relative to other neighboring particles. Defluidization is therefore defined herein as the status of particles having substantially no motion relative to other particles. Typical defluidization phenomena include channeling and xe2x80x9cdead spots. xe2x80x9d Channeling is a phenomenon in which fluids rapidly rise in one area of the bed, sometimes carrying particles with them, and can adversely affect the fluidization of other areas of the bed. Channeling even in a small area often adversely affects fluidization in other areas of the bed. Channeling does not need to extend for the entire height of the fluidized bed but may be present in a short length (such as a few, i.e four, inches in height) and may cover only a small portion of the cross section of the bed. Although in places herein we mention both channeling and local defluidization as types of upsets in fluidized beds, channeling is actually a type of local defluidization, and we include it within the meaning of local defluidization, especially as a type of local defluidization. If any area within the bed, especially within the bottom part of the bed, becomes defluidized and channeled (due to non-uniform radial distribution of fluidizing gas, or too much liquid in the bed, for example), temperature uniformity in the bed could be degraded. Variations in temperature can result in a wide range of product characterization and operational difficulty. If those problems are not corrected in a timely manner, polymer agglomerates can form and an expensive premature shutdown of the reactor may be required. In addition, the product quality can also be adversely altered. Therefore, it is desirable to provide a method to detect and correct local defluidization including channeling, in fluidized-bed polymerization reactors.
A few common examples of local-defluidization prone operations include xe2x80x9ccondensing modexe2x80x9d operation with high levels of condensing, operation with sticky resin particles in the fluidized bed, periods of product transition, and operations with relatively significant variations in operating parameters and particle properties. A reliable method to detect and correct local defluidization including channeling, would allow increased product throughput and improved product quality.
A Japanese patent (JP07-8784) describes the use of a method to detect the xe2x80x9cbed collapsexe2x80x9d of a fluidized-bed granulator due to channeling, wherein emergency controls are employed to temporarily increase gas jetting flow to return the bed to normal fluidization. According to this patent, a sharp decrease in total bed pressure drop in the fluidized bed may indicate that the bed has been switched to a channeling mode of operation. This patent describes the use of elastic wave theory to measure a sharp change in the vibration of the fluidized bed, which may also indicate the presence of channeling. These techniques are used to detect a drastic conversion to channeling flow in the granulation fluidized bed. This may not serve as an adequate warning for fluidized-bed polymerization reactors where the onset of channeling has to be detected before the channeling is fully developed. The techniques of the Japanese patent do not detect channeling early enough to prevent polymerization degradation.
Another patent (U.S. Pat. No. 4,858,144) describes the process of measuring pressure signals to identify when agglomerates and other large particles are present in a fluidized-bed polymerization reactor. It does not detect local defluidization and channeling problems in order to control them.
Griffin et al., in U.S. Pat. No. 5,462,999, suggest the use of a Z function, which is related to gas density, solid density, settled bulk density and fluidized bulk density, to maintain desired fluidization conditions when running the reactor in condensing mode. However, such a Z function targets on the change of the overall bed properties and can not be used to promptly detect local defluidization and channeling.
This invention provides an innovative and simpler way to detect and correct local defluidization and channeling.
This invention provides a method of monitoring, detecting, and correcting local defluidization problems, including channeling, in fluidized-bed polymerization reactors for polymerizing at least one alpha olefin or polymerizable diolefin with other optional copolymerizable monomers, comprising polymerizing the monomers in the presence of at least one polymerization catalyst, optionally using xe2x80x9ccondensing-modexe2x80x9d operation, and also optionally in the presence of an inert particulate material. Our invention is also operable for polymerization of substituted olefins such as styrene, acrylonitrile, and other unsaturated polymerizable monomers mentioned elsewhere herein. Detection of incipient defluidization is conducted by comparing a signal having a value which is a function of currently measured fluidization bulk density with a signal having a value which is a function of a time-averaged baseline of the fluidization bulk density using input data from the same source as the currently measured fluidization bulk density. For certain operations, the time-averaged baseline can also be determined according to historical data selected for its relation to successful production, known as xe2x80x9cgoodxe2x80x9d data. While the functions of the measured bulk density may vary, we prefer two particular types of functions.
This invention provides two preferred criteria for monitoring and detecting an incipient local defluidization and/or channeling, and procedures for returning the reactor to normal fluidization. Both are based on functions of fluidized bulk density. The first criterion recognizes the occurrence of local defluidization and/or channeling when the Channeling Index (hereafter sometimes called xe2x80x9cCindexxe2x80x9d) is larger than about 10%, and the second criterion recognizes the local defluidization and/or channeling when the standard deviation of fluidized bulk density drops more than about 20% from a baseline standard deviation.